The invention relates to removable guide posts for the guide base of a subsea oil or gas installation.
During drilling, a guide base normally is positioned around the conductor housing of a well. The guide base includes upstanding guide posts at the corners of the guide base frame. The lower ends of wire lines or cables are anchored to the guide posts and the upper ends of the wire lines are supported from a floating drilling vessel. After the conductor housing and guide base are installed, a well head, blow out preventers, a Christmas tree and other equipment are guided into position by the wire lines and the guide posts.
Subsea guide bases are usually permanently installed and cannot be removed or repair. If a guide post becomes bent or damaged during use, the drilling operation would be disrupted if it was necessary to retrieve the guide base. In such cases, it is a definite functional advantage to retrieve the damaged post independently. At other times it may be desirable to transfer undamaged guide posts from one well head area of a seabed to another well head area.
Techniques recently have been developed to replace guide posts without removing the guide base from the seabed. These techniques generally require the use of a diver or a removely operated vehicle (ROV). Unfortunately, the equipment designed for subsea removal or guide posts generally includes a complex mechanism having a hydraulically operated split ring or a locking pin. These mechanisms generally do not provide definite visual feedback to the diver or ROV in all the functional modes of operation to determine if the mechanism is or will function properly once the diver or ROV has left the guide base. Another disadvantage of these mechanisms is that they generally require diver or ROV assistance throughout the entire guide post removal sequence, thus necessitating intricate work requirements by the diver or ROV. A ROV in such a situation will, in many cases, have to "dock" or attach itself in a stable manner to the structure in order to have a reasonable change to perform the intricate manipulating tasks required to operate the mechanism.
Modern ROV's have the potential to do many different types of motion or manipulation, i.e., rotation, grasping, lifting and the like, but generally have the ability to exert only minimal amount of force in doing so. This fact has caused a problem to which existing post mechanisms do not adequately address. A mechanism to hold a guide post in place under loads in the thousands of pounds must be "unlatched" or "overridden" by ROV forces generally measured in the tens of pounds.
Accordingly, there remains a need for a simple mechanism allowing replacement of guide posts without removing the guide base from the seabed. The mechanism should require minimal use of a diver or an ROV, should provide visual feedback to the operator, should require minimal force to operate and should require no or minimal operator assistance during the guide post removal sequence.